Inhibition of corrosivity of sulfuric acid on carbon steel



April 25, 1967 c. w. HOORNSTRA ETAL 3,

INHIBITION OF CORROSIVITY 0F suwumc ACID ONQCARBON STEEL Filed Aug. 24, 1965 7 Sheets-Sheet 5 TEMPERATURE v5 CORROSION R475 40 MIL cop oso/wzrn? PROBE //v /NH/B/TED 70 H2 50 4-2 u/v/r five/age corros/on rafe MPY fla/ua/ //'me in days INVENTORS Pau/ R. Hand) C/ag/on 0V. Hoorns/ra April 25, 1967 c. w. HOORNSTRA ET AL 3,316,179

INHIBITION OF CORROSIVITY OF SULFURIC ACID 0N CARBON STEEL Filed Aug. 24, 1965 7 Sheets-Sheet 6 CORROSION RATES OFMY/LD 375a //v 70 000/5005 H2504 lNH/B/TEO flCCORD/NG r0 7'/-/ //WN770N 0+ a Fe Way/2 /05.5' my.

0 l l L l l 0 2 4 6 8 l0 l2 Time-day's CORPOS/ON R9755 0F M/LD STEEL //V 70 zQQUEOl/S H 59 //VH/B/ TED IQCCORD/NG TO THE lNl/fNT/ON O l l I l 1 0 2 4 6 a l0 l2 T/me-aays INVENTORS.

ATTORNEY United States Patent 3,316,179 INHIBITION OF CORROSI"ITY F SULFURIC ACID 0N CARBON STEEL Clayton W. Hoornstra and Paul R. Handt, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Aug. 24, 1965, Ser. No. 483,029 13 Claims. (Cl. 252-147) Metals Handbook, 8th Edition, page 62. (1961), published by the American Society for Metals wherein carbon steels usually bear the identity of the Society of Automotive Engineers (S.A.E.) number in the 1000 series.

Sulfuric acid is among the most extensively used of chemical reagents. For its use, it is frequently necessary that it is brought into contact with steel equipment, apparatus, vessels, valves, conduits, pumps, tanks, and steel articles in general. Sulfuric acid is corrosive. The corrosion of such articles is undesirable, generally, the loss of metal resulting in a weakening of the walls of vessels and of structural members and in a lessening of the number of years of service. Such undesirable effects have prompted a diligent quest to discover more effective ways to reduce the rate and extent of such corrosion.

A recent and expanded use of relatively concentrated sulfuric acid is in the pickling of magnesium and manganesium-base alloy articles wherein it is necessary to store, move, and treat magnesium and magnesium-base alloy articles in steel tanks, conduits, and vessels. These tanks, conduits, and vessels are commonly made of car- 'bon steel which may be identified by an S.A.E. number between 1006 and 1095. Thesulfuric acid used for magnesium pickling is usually at least about 55 percent by weight aqueous H 80 solutions and invariably at least 40 percent by weight; it is preferably at least about 65 percent by weight H 80 Among the suggested ways to reduce the corrosion of steel due to the attack thereof by corrosive acidic liquids in contact therewith has been that of admixing certain compounds known as inhibitors, e.g., arsensic, nitrogen, and sulfur compounds.

Although the use of such compounds has been at least partially successful in the reduction of undesirable corrosion in some instances, e.g., the corrosive effects of hydrochloric acid, they have not been satisfactory for use in lessening the corrosivity of relatively highly concentrated sulfuric acid on carbon steel.

A need representing extensive potential savings has existed throughout the industrial age for a more effective and more economical method of inhibiting the corrosivity of relatively highly concentrated aqueous solutions of sulfuric acid on carbon steel.

The invention contemplates a method of treating carbon steel and further contemplates a method of inhibiting the corrosivity of aqueous H 80 solutions, having a concentration of at least about 40 percent by weight H 80 on carbon steel vessels and articles in contact "ice chromic compounds andfzgpmimgnd chromium metal preferably supplemente by a nickelous gompound gr, nickel metal in an amount su crent to yield at least about 0.01 percent by weight of Al ions when aluminum or an aluminum compound is employed, at least about 0.005 percent by weight of Cr, ions when a chromium or a chromic compound is employed, and preferably at least 0.01 percent of Ni ions when a nickel compound or nickel metal is employed. It is also preferable to add a ferrous compound or iron metal to the aqueous H solution to provide Fe ions.

Herein, when the term Cr ions is used, the Cr+++ or trivalent chromium ions are intended; when the term Al ions is used, Al+++ or trivalent aluminum ions are in tended; when the term Ni ions is used, Ni++ or the divalent ions are intended; when the term Fe ions is used, Fe++ or divalent ions are intended. It is understood that both the metal and one or more compounds of the metals desired may be employed together.

A second aspect of the invention consists essentially of treating carbon steel by contacting it with the aqueous solution prepared as in the method described immediately above.

Fe ions need not be added but may be formed in the H 80 solution, containing either the aluminous compound or the chromic compound (preferably with the nickelous compound) on a steel vessel containing the acid by corrosion of the surface of the confining steel wall by the acid until sufficient FeSO, has been formed to saturate the acid. This mode has obvious disadvantages.

Accordingly, the preferred embodiment of the invention is to admix a ferrous compound or iron metal and either the Cr ion source or the Al ion source with the aqueous H 50 solution. The presence of the Fe ions avoids formation thereof at the expense of the steel vessel, conduit, or members in contact therewith.

It is desired that there be present (other than by acidic attack of the walls of the vessel) suflicient Fe ions to provide at least 0.02 percent by weight of the acid. Any iron containing source may be used which ionizes since any Fe++ ions will yield FeSO, up to the point of saturation.

A second embodiment of the invention is to admix a ferrous compound or iron, one of either the aluminous or chromic compounds or metals or a mixture of all, and a nickelous compound or nickel metal to provide: sufficient of the iron to result in substantial saturation of FeSO at least 0.01 percent by weight of Al ions, at least 0.005 percent by weight of Cr ions, and at least 0.01 percent by weight of Ni ions, based on the weight of the aqueous solution of H 80 Another embodiment of the invention is to provide all four types of the above-named compounds or metals, which may include some compounds and some metals i.e.,

by admixing compounds or metal to provide Fe, Al, Cr or Ni ions in amounts sufficient to provide at least the amounts of the metal ions set out above.

The temperature of the aqueous H 80 to be inhibited is recommended to be not greater than about 160 F. and more desirably not greater than about 140 F. In the practice of the invention, greater amounts of the ferrous compound are preferred to protect carbon steel surfaces, which are in contact therewith, at the higher temperatures and/or at the lower concentrations of H 80 than at lower temperatures and/ or at the higher concentrations within the temperature and concentration ranges above stated. For example, when the corrosivity of an aqueous solution of about 55 percent H 50 is to be inhibited at about 130 F. or less, up toward about 3.0 percent by weight of Fe ions is desired. On the other hand, when the corrosivity of a 70 percent aqueous solution of H2804 is to be inhibited at about 130 F the amount of Fe ions necessary is only about 0.3 percent. Although lower temperatures and/or higher concentrations of aqueous H 50 solutions require less Fe ions for satisfactory inhibition, neither the temperature nor the concentration of the H 50 has an appreciable direct effect on the amount of the other metallic ions needed for satisfactory metal protection.

It is preferred that both Ni ions and at least one of Al or Cr ions be provided because, as hereinafter shown, either Cr or Al ions give comparatively long protections but show some delay in becoming effective, whereas Ni ions show a prompt inhibitory effect, but such effect is not so effective over a long-term exposure'as is that resulting from either the Al or Cr ions. Although corrosion of any of the carbon steels is inhibited by the practice of the invention, it is not fully satisfactory for use against corrosion of such special steels as stainless steel.

The invention has highly inhibitory effects on the corrosivity of sulfuric acid of the concentrations stated, not only when the acid is substantially stationary, but: when it is being moved at high velocity, when being subjected to high turbulence or agitation, or when subjected to continuous contact with oxygen as when air is bubbled therethrough. The corrosivity of an acid in contact with air, particularly when air is entrained therein, is well known to be accelerated.

The following examples are illustrative of the practice of the invention. Tests runs, not in accordance with the invention were also made for comparative purposes. The examples and comparative tests were carried out on mild carbon steel, designated S.A.E. 1018, by placing coupons of the steel, having a total exposed surface of 0.45 square decimeter, in selected aqueous solutions of 70 percent by Weight H 50 each containing ferrous compounds admixed therewith, with or without additional metallic ions present, as stated in the individual examples and comparative tests. In the examples all the percentages are for amounts by weight.

RUN A FOR COMPARATIVE PURPOSES Above designated steel coupons were placed in the 70 percent aqueous solution of H 80 at a temperature of 120 F. The solution was agitated and aerated by passing air upwardly therethrough. The coupons were left in the H 80 solution for varying periods of time, viz.,

3 hours, 1 day, 9 days, and 14 days. The average rate of corrosion in mils per year (m.p.y.) for each coupon was ascertained. (One mil equals 0.001 inch.) The m.p.y. values were obtained by weighing the coupons, calculating the weigh-t loss, and dividing the weight loss by the area to ascertain the thickness of metal in mils which would be lost in one year at the average rate of corrosion existing for the period of time elapsing between examinations of the coupons. (The procedure employed is discussed in more detail, infra.) The corrosion rates in m.p.y. are plotted on the ordinate and the time in days in percent by weight of Fe (formed as FeSO; in the solution) on the abscissa of the graph comprising FIG- URE 1. The times of examination are shown on the curve itself.

Reference to FIGURE 1 shows that the rate of corrosion decreases rather rapidly until the concentration of Fe ions in solution reaches about 0.034 percent by weight (the estimated saturation point for FeSO, at that temperature) and thereafter the rate of corrosion rose only slightly. The rate of corrosion thus obtained, viz. between about 50 and 60 m.p.y., as there shown, is too great. Accordingly, sulfuric acid cannot be satisfactorily continued in contact with steel surfaces without protection of the steel.

Example 1 A series of four test runs was made, one for comparative purposes and three in accordance with the practice of the invention wherein a 70 percent aqueous solution of H 50 containing metallic inhibitory ions, was employed at 124 F. Mild steel coupons of the type and having the surface area of those employed in the comparative run above were immersed in the so prepared 70 percent aqueous solution of H 50 for a specified period of time. The rates of corrosion were ascertained by periodic examination of the coupons, consisting of removing, drying, and reweighting each coupon, and calculating the rate of corrosion in mils per year (m.p.y.) for the average time (expressed in days) between each examination. The results are shown as curves A, B, C, and D of FIGURE 2 wherein the ordinate is the average rate of corrosion in m.p.y. and the abscissa is the average length of time of immersion in days, between succeeding examinations. For example, if an examination was made on the fourth day and another on the tenth day, then or 7 days determines the positions of the point on the curve along the abscissa.

In the first of the series of this example (for comparative purposes), designated Run A, FeSO -7H O was added to a portion of the H 50 solution in an amount suflicient to provide 0.1 percent of Fe ions therein. A steel test coupon was immersed therein, periodically, removed, and the average weight loss ascertained. The results are plotted on curve A of FIGURE 2.

To a second portion of the 70 percent aqueous solution of H were admixed suifieient FeSO -7H O to provide 0.1 percent of ions and sufiicient CrCl to provide 0.1 percent of Cr ions therein. A steel coupon of the type employed above was immersed therein, the corrosion thereof periodically measured, and the rate of corrosion in mils per year calculated as above. The results are plotted as curve B of FIGURE 2.

In the third of the series as part of Example 1, there were admixed, with another portion of the 70 percent aqueous solution of H 80 sufficient FeSO -7H O to provide 0.1 percent of Fe ions and sufficient NiSO -7H O to provide 0.045 percent of the Ni ions therein. A steel coupon of the'type above employed was immersed therein and the effect on the corrosivity of the thus inhibited H 80 solution thereon ascertained, as above, and the results plotted as curve C on the graph of FIGURE 2.

To a fourth portion of the 70 percent H 80 there were admixed sutficient FeSO '7I-I O to provide 0.1 percent of the Fe ions and sufiicient Al (SO -18H O to provide 0.05 percent of Al ions. A steel coupon of the type above tested was immersed therein and examined as above. The results are shown on the curve designated D of the graph of FIGURE 2.

Reference to the results shown in FIGURE Zsupports the following conclusions: Curve A shows that the presence of Fe ions, whether formed by corrosion of the steel coupon (or similarly by the corrosion of walls of a confining steel vessel) or whether provided directly by admixing a ferrous compound inhibitor therewith, decreases the rate of corrosion to a rate of somewhat lower than that shown in FIGURE 1, but comparable thereto and clearly unsatisfactory. Curve A of FIGURE 2 indicates that some period of time appears necessary for an equilibrium constant rate to be attained in the H 80 solution but, that after such equilibrium rate had been attained, the inhibition of corrosion is not substantially better than that which was found to occur when the Fe ions were provided by corrosive attack on the coupon.

Curve B of FIGURE 2, wherein both Fe ions and Cr ions are present, shows that the rate of corrosion decreases until an average rate of corrosion of only 5 mils per year occurs. This clearly shows the inhibitory properties of the combination of Fe and Cr ions.

Curve C of FIGURE 2, wherein both Fe and Ni ions are present, shows superior improvements for a short time but one which is not so effective as that provided by the presence of both Fe and Cr ions over an extended period of time.

Curve D, wherein both Fe and Al ions were employed, shows a somewhat slightly quicker inhibitory effectiveness at first, over the combined effect of Fe and Cr ions, but, in general provides an inhibitory effect which is comparable to that of the Fe and Cr ions together. They both perform very satisfactorily.

Example 3 This example was run to show the preferred amounts of the inhibitory metals to employ. A 70 percent aqueous solution of H 50 at 124 F. was placed in a suitable vessel and FeSO -7H O in an amount sufficient to provide 0.1 percent of Fe ions was admixed therewith. In separate containers CrCl was admixed therewith in an amount sufficient to provide, in the three successive containers, 0.01 percent, 0.02 percent and 0.1 percent of Cr ions therein. Coupons of the type employed in the examples above were placed in each of the three containers and the rates of corrosion ascertained. The results in average corrosion in m.p.y. are shown on the graph of FIGURE 3.

Reference to FIGURE 3 shows that the inhibitory properties of the different concentrations of Cr ions (although showing some differences in the first couple of days) provide comparable satisfactory inhibitory effects. This indicates that but a small amount of Cr ions is required in the practice of the invention.

Example 4 This example was run to show the effects of fixed amounts of Fe and Ni ions supplemented by varied amounts of Cr ions. A solution of 70 percent aqueous H 50 as employed in the above example, at a temperature of 124 F., was placed in a satisfactory container and sufficient FeSO -7H O and NiSO -7H O admixed therewith to provide, respectively, 0.1 percent of Fe ions and 0.04 percent of Ni ions by Weight of the H 50 solution. Portions of the solution thus prepared were placed in three containers. To each of these containers was admixed sufficient CrCl to provide Cr ions in the amounts of 0.005 percent in one container, 0.01 percent in the second container, and 0.02 percent in the third one, based on the weight of the aqueous H 50 solution. Mild carbon steel coupons of the type employed above were immersed in each of the containers and the rate of corrosion determined. The results are shown in the graph of FIGURE 4.

Reference to FIGURE 4 shows that the effect on corrosivity of a 70 percent by weight aqueous solution of H 50 inhibited as was done herein by the presence of Fe, Cr, and Ni ions accelerates the inhibitory effect of the Cr ions. It shows that the higher concentration of Cr ions provides a slightly better inhibitory effect (except for a slight delayed effect at first) than the lower concentrations thereof.

During this example the parts per million of Cr trivalent ions were also determined over a period of 15 days to ascertain whether or not they become depleted. Tests on the first, sixth and fifteenth days showed that the chromium ions remained substantially constant.

Example 5 This example was run to show the effects of varying the amount of both the Ni and Cr ions. Both Ni and Cr ions were provided in a 70 percent aqueous H solution containing 0.1 percent Fe ions similarly as the preceding example. Both the Ni ions and the Cr ions were varied, in three tests, in amounts of 0.01 percent, 0.02 percent, and 0.04 percent in the following manner: 0.01 percent Cr and 0.01 percent Ni; 0.02 percent Cr and 0.02 percent Ni; and 0.04 percent Cr and 0.04 percent Ni. The temperature employed was 124" F. The results are shown in the graph of FIGURE 5.

Reference to FIGURE 5 shows that similar inhibitory effects are obtained in all concentrations of the Ni and Cr ions employed, but that when 0.01 percent of each was employed, there was less consistent inhibitory effect, perhaps due to less effective uniform distribution of the smaller quantities throughout the body of acid.

Example 6 This example was performed to show the effects of the presence of both Cr and Al ions to inhibit H 80 attack on carbon steel. A 70 percent aqueous solution of H 80, as in the above examples, at a temperature of 124 F., was placed in a container and 0.1 percent Fe ions provided as above. The solution thus prepared was divided into three portions and placed in separate containers. Into each of these containers were admixed suflicient CrCl and Al (SO -18H O to provide the following amounts of each in the containers: 0.01 percent Cr and 0.01 percent A1 ions in one container; 0.02 percent Cr and 0.02 percent Al ions in a second container; and 0.04 percent Al and 0.04 percent Cr ions in the third container. The inhibitory effects of the presence of these metals, in combination, are set out on the three curves comprising FIGURE 6.

Reference to FIGURE 6 shows that highly satisfactory results are obtained in all three runs. There appears to be no particular advantage in using the higher concentration of inhibitors during the period of time in which this example was run.

Example 7 The runs of this example were made because, among the uses of aqueous H 50 solutions in concentrations of at least about 55 percent, is that of pickling magnesium and magnesium-base alloy articles. When an aqueous H 50 solution is employed for this purpose, there is commonly admixed therewith, a smaller but effective amount of a brightening agent, one of those recently discovered which is highly effective being hexamethylenetetramine. In such magnesium pickling operation, some magnesium sulfate is formed and is permitted to accumulate to saturation in the pickling solution. Accordingly, this example was run to simulate typical pickling operations wherein MgSO was present. Two solutions of 70 percent by weight aqueous H 80 were prepared and evaluated (for the purpose of comparing the effectiveness of the invention in the presence of the brightening agent and in the absence thereof). Sufficient MgSO was admixed with each solution of the 70 percent aqueous solution of H 80 at 124 F. to saturate substantially each solution. There was then admixed with each solution sufficient of the ferrous, chromic, and nickelous salts employed above to provide 0.03 percent Fe, 0.02 percent Cr, and 0.04 percent Ni. The solution showed by test to be about 0.1 normal MgSO The solution was divided into two portions. To one of the solutions there was admixed hexamethylenetetramine in an amount sufficient to provide 50 grams thereof per liter of solution. Coupons of S.A.E. 1018 steel, as above used, were immersed in each solution and the corrosion rate ascertained.

The results are shown in the curves of FIGURE 7, as there designated.

Reference to FIGURE 7 shows that the presence of the combination of the metals, in accordance with the practice of the invention, is highly effective to inhibit the corrosion of carbon steel in the presence of either magnesium sulfate alone or in the presence of both hexamethylenetetramine and magnesium sulfate.

Example 8 This example was conducted to show the efiicacy of the practice of the invention for a prolonged period of time.

To one portion, designated A of a 70 percent aqueous solution of H 50 there was admixed only FeSO -7H O in an amount suflicient to provide 0.03 percent of Fe ions by weight of the H 80 solution.

To a second portion, designated B, of a 70 percent aqueous solution of H 50 there was admixed FeSO 7H O in an amount suflicient to provide 0.03 percent of Fe ions, CrCl in an amount sufiicient to provide 0.02 percent of Cr ions, and NiSO -7H O in an amount sufiicient to provide 0.04 percent Ni ions therein. Coupons of the type and surface area used above (viz., S.A.E. 1018 carbon steel having a total surface area of 0.45 square decimenter) were immersed in each of the A and B aqueous H 80 solutions so prepared. Six test coupons were employed in portion A and six in portion B, thusly obtaining twelve separate test results in the aqueous H 80 solutions.

The coupons were allowed to remain in the test compositions at varying times and then removed, dried, and weighed to determine weight loss and the rate of corrosion in mils per year calculated. Table I shows the percent of the various metal ions in the 70 percent H 80 acid solutions, the time of immersion, and the rate of COITOSlOIl.

TABLE I S.A.E. 1018 coupons, 0.45 square decimeter in area 70 percent by weight aqueou solution of H180; Time Weight Corrosion in H1804 loss in rate in solutions milligrams mils/year in days 0.03 percent Fe++ only" 1 7l. 5 29. Do 2 141 28. 6 270 18. 3 l4 1, 184 34. 4 35 3, 555 (l. 4 Do 45 6, 174 56. 0 0.03 percent Fe, 0.03 percent Cr and 0 04 percent N i+ l 118 48. 0 D 2 159 32. 4 6 251 17. 0 14 331 9. 6 35 488 5. 7 45 551 6. 0

The results shown in Table I are shown graphically in FIGURE 8 wherein the weight loss in milligrams is plotted against the time in days. The continuous line 'curve represents the inhibition of corrosion in accordance with the invention and the broken line in the presence of a. ferrous metal compound only. It is clearly shown that the rate of corrosion was markedly inhibited in accordance with the practice of the invention.

Example 9 This example was run to show the eflect of temperature on the inhibition of corrosivity of metal ions according to the invention in a relatively highly concentrated aqueous H 50, solution. The example consists of a series of corrosivity values of a 70 percent by weight aqueous solution each containing 0.03 percent Fe, 0.02 percent Cr, 0.04 percent Ni ions, provided by adding the ferrous sulfate, chromic chloride, and nickelous sulfate hydrates as employed hereinabove. The corrosion rates were ascertained at the temperatures stated hereinbelow, by use of an electric recording instrument known as a Magna Corrosometer, L-2 Model, made by the Magna Corporation, Santa Fe Springs, Calif.

The Corrosometer is used to measure corrosion rates by electrical means. A wire measuring element is exposed to the corroding solution. This wire is part of a resistance bridge circuit. As the wire corrodes, its resistance changes and thus the bridge becomes unbalanced. When the bridge is rebalanced a new reading is obtained on the Corrosometer. The readings are recorded in absolute units. By measuring the increase in units as shown by readings per time interval, a corrosion rate (in mils per year) can be calculated. For instance, for a 40 mil wire element, a change of 1 unit per day is equivalent to 3.65 mils per year corrosion rate. The L-2 automatic recording Corrosometer measures changes in units continuously and plots them on a recorder. The corrosion rate can be calculated by measuring the slope of the line resulting from plotting the readings of the recorder.

The first readings of the corrosion due to the H solution were taken at 126 F. Thereafter the temperature of the solution was raised successively to F., F., and F., and a series of corrosion rate readings obtained at each temperature. It is to be borne in mind in appraising the findings of this example that the same samples were continuously and uninterruptedly employed throughout the example and were not removed from the inhibited H 80 solution at the temperature changes. The rate of corrosion due to the presence of the inhibiting ions decreases over the period of the test at a given temperature, due to a phenomenon inherent in the invention which appears to be due to the build-up of a protective film on the samples. Accordingly, the inhibition of corrosion of the samples at the higher temperatures appears to be better than it actually is because it has the benefit of the resistance to corrosion which accrued during the lower temperature in the inhibited H 80 solution.

The results of this example are plotted on curves, identified by the temperatures, in FIGURE 9.

Reference to FIGURE 9 shows that the higher temperatures are less satisfactory. It is recommended that the H 50 solution to be inhibited, be kept as near room temperature as possible; it is preferred that it not be permitted to rise above about 160 F.

COMPARATIVE RUN B A number of organic inhibitors known to be among those considered more or less effective to inhibit the attack of dilute sulfuric acid on carbon steel were admixed with 70 percent aqueous H 80 solution containing 0.04 percent Fe ions and the corrosion rate determined as above over a period of 5 days. Among the organic inhibitors tried were aliphatic alcohols, amines, sulfur compounds, and alkoxy-substituted oxybenzenes. Rates of corrosion in mils per year were obtained which were definitely inferior to those obtained by the practice of the invention and, in general, unsatisfactory for use to inhibit corrosion of iron or steel in contact with such aqueous H 80 solutions.

In the practice of the invention, it is to be borne in mind that any source which yields Fe ions, Cr ions, A1 ions, or Ni ions in the aqueous H 80 solution is fully satisfactory. Any compound which yields such ions in the aqueous H 80 solution results in the formation of some sulfate of the metal added, e.g. FeSO Cr (SO A1 60 or NiSO all of which are adequately soluble to provide the minimum of metal ions required for the practice of the invention. The anion of the compound employed is unimportant since the metal portion thereof provides the protection.

To illustrate the solubility of various compounds of iron, chromium, aluminum and nickel solubilities were ascertained by the following procedure.

I Example Thirty grams of each of the compounds listed in Table H below were admixed with milliliters of 70 percent by weight aqueous H in individual containers. The solutions so treated were observed and analyzed after about eight hours at about 75 F. (room temperature). The percent by weight of metal ions dissolved are shown in Table II.

References in Table II show that various salts, oxides, and hydroxides of the desired metals to be employed 1 in the practice of the invention are adequately soluble in the aqueous H 80 solution to provide an ample source of the protective metal ions.

are also shown graphically in FIGURE 10 and the results of tests numbered 2, 4 and 7 are shown graphically in FIGURE 11. In each of the figures, the time in days is plotted along the abscissa and the loss of weight in milligrams is plotted along the ordinate.

The tests of this example were conducted as follows:

Mild steel coupons designated S.A.E. 1018, having a total exposed surface of 0.45 square decimeter, were immersed in the acid in each container and the corrosion rate ascertained over a period of days. The rate of corrosion was ascertained as hereinabove described when providing the metal ions by adding metal compounds. Broadly the method consists of obtaining the factor for converting loss of metal in milligrams per square decimeter of exposed surface per day to depth of corrosion in mils of metal loss per year. This method of computation is described in the literature, e.g. Corrosion-Processes, Factors, Testing, pages 35 and 36, published by the International Nickel Company (1951). The method of com- Since the total exposed area of each steel coupon employed was 0.45 dmF, the factor 0.183/0.45 was used to give the conversion factor in the following equation:

Total weight loss 0.l83 weight loss in mils/year Number of days 0.45 (m.p.y.) The number of days of immersion of the steel coupons and the corrosion in mils per year, as so calculated. are shown in Table 111.

TABLE IIL-RATE OF CORROSION OF MILD STEEL IN 3,860 GRAMS OF 70 PERCENT H1804 ACID, INHIBITED BY SELECTED METAL IONS AC- CORDING TO THE INVENTION To show that aluminum, chromium, nickel and ferrous metals may be used to provide the desired metal ions, the following example was conducted.

Example 11 Reference to Table III and to FIGURES l0 and 11 shows that strips of pieces of ('1) each of Al or Cr metal alone, (2) either Al or Cr metal together with Ni metal, (3) Al metal with Ni metal and a ferrous compound, or (4) an Al compound with Ni metal imparts marked improvement in resistance of steel to the corrosive action of H at relatively high concentrations in water. By reference to Comparative Run A supra (and as shown on FIG. 1), it is seen that, in the absence of metal ions to inhibit corrosion in accordance with the invention, the average rate of corrosion over a nine year period was about fifty mils per year.

1 1 Example 12 The following tests were conducted to show that the invention can be satisfactorily conducted for protection of steel against corrosion caused by various concentrations of aqueous H 50 by weight, within the acid concentration signified, but is not satisfactorily effective for less concentrated H 50 solutions.

(1) Three concentrated sulfuric acid solutions were prepared, viz. 35 percent, 55 percent and 70 percent by weight. Sufficient FeSO -7H O was admixed with each of the sulfuric acid solutions so prepared to saturate the solutions therewith, the amounts of Fe++ ions thereby provided being: 2.3 percent Fe++ in the 35 percent H 80 0.37 percent Fe++ in the 55 percent H 80 0.03 percent Fe++ in the 70% H 80 (2) To each of the concentrated sulfuric acid solutions containing the FeSO -7H O were then admixed sufiicient CrCl to provide 0.02% of the Cr+++ ions and sufiicient NiSO -7H O to provide 0.04% of the Ni++ ions, based on the weight of the H 80 solution.

(3) Equal size carbon steel coupons (having a total exposed surface of 0.45 dm?) were weighed and placed in each of the thus prepared inhibited concentrated sulfuric acid solutions and the rate of corrosion of the coupons observed. After 7 days the coupon was removed from the 35% concentrated sulfuric acid, washed, dried, and weighed and the rate of corrosion calculated. It was found to be 247 mils per year. After 50 days, the coupon in the 70% inhibited concentrated sulfuric acid was similarly removed, washed, dried and weighed and the rate of corrosion found to be at the rate of 2.9 mils per year. After 60 days, the coupon in the 55% inhibited concentrated sulfuric acid was similarly removed, washed, dried and weighed and the rate of corrosion found to be 5.2 mils per year.

-In the graph forming FIG. 12 of the drawing, corrosion rates in mils per year of the coupons are plotted on the ordinate and the H 80 solution concentration employed are plotted on the abscissa. The graph shows, by horizontal lines, the relatively high conventional rate of corrosion desired in etching and pickling operations and the much lower corrosion rate which is acceptable for inhibiting corrosion of steel vessels, conduits, and equipment containing H 80 solutions. FIGURE 12 also shows that an inhibitor to the corrosion attack of sulfuric acid on storage vessels and equipment is not acceptable until the rate of corrosion is not appreciably greater than about 25 mils per year. The graph also shows that a pickling bath is not acceptable for the intended purpose unless its rate of attack (corrosion) on the metal being pickled is not appreciably less than about 84 mils per year. This shows that a bath suitable for etching or pickling is not acceptable as a corrosion inhibiting bath.

It can readily be seen from an examination of FIGURE 12 that the efficiency of the invention is not discernible until at least about a 40% concentrated sulfuric acid is being inhibited when the inhibition becomes appreciable. Thereafter, with increasing concentration of the sulfuric acid solution employed, the effectiveness of the invention .becomes much more impressive.

The examples show that the pressure of the selected metal ions in 40% or more concentrated H 80 acid very from the class consisting of chromic and aluminum compounds and chromium and aluminum metal and mixtures thereof in an amount sufficient to yield by weight in said solution at least about 0.005 percent by weight of trivalent chromium ions when the metal ion-yielding source yields chromic ions and at least about 0.01 percent of trivalent aluminum ions when the metal ion-yielding source yields aluminum ions, and (3) the balance substantially water.

2. The method according to claim 1 wherein said aqueous composition contains an additional metal ion-yielding source material selected from the class consisting of ferrous compounds and iron metal in an amount sulficient to yield at least about 0.02 percent by weight of divalent Fe ions in solution.

3. The method according to claim 1 wherein said aqueous composition contains an additional metal ion yielding source selected from the class consisting of nickelous compounds and nickel metal in an amount sufiicient to yield at least about 0.01 percent by weight of divalent Ni ions in solution.

4. The method according to claim 1 wherein said composition contains both chromium ions and aluminum ions.

5. The method according to claim 4 wherein said composition also contains a divalent nickel ion-yielding source in an amount suificient to yield at least about 0.01%, by weight of said aqueous composition, of divalent Ni ions.

6. The method according to claim 5 wherein said composition also contains a divalent iron ion-yielding source.

7. The method according to claim 1 wherein said composition contains at least about percent by weight of H 80 8. The method of inhibiting the corrosive effects of an aqueous H 80 solution, having a concentration of at least about 40 percent by weight and a temperature not greater than about 160 F., on carbon steel in contact therewith, which consists essentially of admixing with said H 80 solution a metal ion-yielding source material from the class consisting of chromic and aluminum compounds and chromium and aluminum metal and mixtures thereof in an amount suflicient to yield in :said H 80 solution at least about 0.005 percent by weight of trivalent chromium ions when a chromic ionyielding source is employed and at least about 0.01 per cent of trivalent aluminum ions when an aluminum ionyielding source is employed.

9. The method of inhibiting the corrosive efiects of an aqueous H 80 solution having a concentration of at least about 40 percent by weight and a temperature not greater than about 160 F., on carbon steel in contact therewith, which consists essentially of admixing with said H3504 solution sufficient Fe ion-yielded source material to provide at least about 0.02 percent by weight of divalent Fe ions and a metal ion-yielding source other than iron selected from the class consisting of chromic and aluminum compounds and chromium and aluminum metals in an amount sufficient to provide at least about 0.005 percent by weight of trivalent Cr ions when a chromic ion-yielding source is employed and at least about 0.02 percent by weight of trivalent Al ions when an aluminum ion-yielding source is employed.

10. The method according to claim 9 wherein the Fe ion-yielding source is added in an amount sufficient to saturate substantially the aqueous H solution.

11. The method according to claim 9 wherein a divalent nickel ion source material is admixed with the aqueous H 50 solution in an amount sufficient to provide at least about 0.01 percent by weight nickel ions in solution.

12. The method according to claim 10 wherein both the aluminum and chromium trivalent ion-yielding source material are admixed with the H 80 solution in an amount sufiicient to provide at least about 0.01 percent by weight of trivalent chromium ions and at least about 0.02 percent by weight of aluminum ions.

13. The method according to claim 10 wherein a divalent nickel ion-yielding source material is admixed with the aqueous H 50 solution in an amount sufficient to provide at least about 0.01 percent by weight of nickel ions.

References Cited by the Examiner UNITED STATES PATENTS LEON D. ROSDOL, Primary Examiner. 1o ALBERT T. MEYERS, Examiner,

I. T. 'FE'DIGAN, Assistant Examiner. 

1. THE METHOD OF TREATING CARBON STEEL WHICH CONSISTS ESSENTIALLY OF CONTACTING SAID STEEL WITH AN AQUEOUS COMPOSITION CONSISTING ESSENTIALLY OF (1) BETWEEN ABOUT 40 PERCENT BY WEIGHT AND THE POINT OF SATURATION OF H2SO4, (2) A METAL ION-YIELDING SOURCE MATERIAL SELECTED FROM THE CLASS CONSISTING OF CHROMIC AND ALUMINUM COMPOUNDS AND CHROMIUM AND ALUMINUM METAL AND MIXTURES THEREOF IN AN AMOUNT SUFFICIENT TO YIELD BY WEIGHT IN SAID SOLUTION AT LEAST ABOUT 0.005 PERCENT BY WEIGHT OF TRIVALENT CHROMIUM IONS WHEN THE MEATL ION-YIELDING SOURCE YIELDS CHROMIC IONS AND AT LEAST ABOUT 0.01 PERCENT OF TRIVALENT ALUMINUM IONS WHEN THE METAL ION-YIELDING SOURCE YIELDS ALUMINUM IONS, AND (3) THE BALANCE SUBSTANTIALLY WATER. 